1. Field of the Invention
This invention relates to a fixed-focus imaging lens suited to installation in a small-sized imaging apparatus such as a digital camera using an imaging device of a CCD (Charge Coupled Device), a CMOS (Complementary Metal Oxide Semiconductor), etc., or a cameral using a silver film.
2. Description of Related Art
In recent years, a digital still camera (hereinafter simply referred to as “digital camera”) for enabling image information of a photographed landscape, portrait, etc., to be input to a personal computer has sprung into wide use with the widespread proliferation of a personal computer into ordinary households, etc. A module camera for image input (mobile module camera) has often been installed in a mobile telephone with higher functionality of a mobile telephone.
In the imaging apparatus, an imaging device of a CCD, a CMOS, etc., is used. Such an imaging apparatus has been very miniaturized as the entire apparatus because of recent advance in miniaturization of the imaging device. Providing the imaging device with a larger number of pixels has also advanced and higher resolution and higher performance have been intended.
For example, imaging lenses described in the following documents are available as those used with such a miniaturized imaging apparatus. JP-A-2002-221659, JP-A-2004-302058, JP-A-2005-173319, JP-A-2005-227755 and JP-A-2005-292235 describe imaging lenses each made up of three lenses.
As described above, the recent imaging device has advanced in miniaturization and a larger number of pixels and accordingly high resolution performance and a compacted configuration are demanded particularly for the imaging lens for a digital camera. On the other hand, formerly the cost performance and compactibility were mainly demanded for the imaging lens of a mobile module camera, but recently providing the imaging device with a larger number of pixels has also tended to advance in the mobile module camera and requirement for the performance has also become high.
Thus, development of a wide variety of lenses totally improved in the cost, image formation performance, and compactibility is demanded; for example, development of a low-cost and high-performance imaging lens intended for installation in a digital camera in the performance aspect while ensuring compactibility for enabling installation of the imaging lens in a mobile module camera is demanded.
For such requirements, for example, it is possible to adopt three lenses as the number of lenses (three-lens configuration) for compaction and cost reduction and aggressively use an aspheric surface for higher performance. However, adopting an aspheric surface is disadvantageous for manufacturability and easily results in an increase in the cost although it contributes to compaction and higher performance. Thus, to use the aspheric surface, manufacturability needs to be considered sufficiently. Generally, a four-lens configuration would easily result in upsizing and an increase in the cost although it is advantageous for performance improvement as compared with the three-lens configuration. The lenses described in the documents above adopt a three-lens configuration and use an aspheric surface, but are insufficient from the viewpoint of compatibility between the image formation performance and compactibility.